The Situational Rule for SBR01 does not seem to prohibit ommitting a sequence - for example, I don't see anything that would make sending "T" in 2000B SBR01 and "P" in 2320 SBR01 with no loop 2320 having SBR01="S". My guess is this was deliberate, since we can't rule out "Payer 1" and "Payer 3" potentially covering a given service type, which "Payer 2" never pays for. Can you please confirm? Thanks.
Yes, it is not required to send a Secondary payer in sequence in order to send a Tertiary payer loop. If it is known that the secondary payer is not involved in paying on this claim in the scenario you described, it is not required to send a 2320 loop for the secondary payer.